1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer or a copier, of an electrophotographic system or an electrostatic recording system, and relates in particular to a color image forming apparatus wherein a plurality of image forming means are arranged vertically.
2. Related Background Art
Recently, speed, function and color recording has been developed for image forming apparatuses of an electrophotographic system, and various types of printers and copiers are available on the market.
Above all, image forming apparatuses of an in-line system, in which image forming means for plural colors are disposed in series and toner images are sequentially multi-layer transferred, can form color images at high speed, and it is anticipated that in the future color printers incorporating such in-line type image forming apparatuses will be the primary color printers.
An in-line type image forming apparatus employs a method whereby a recording medium (transfer material) is borne on the surface of belt-shaped conveying means, for transporting the recording medium, while toner images are sequentially transferred to the recording medium to form a multi-layer color image. According to this method, the configurations of the apparatuses can be roughly sorted into two types, depending on the direction in which the recording media are conveyed.
In one configuration, as is shown in FIG. 9, process stations 8M, 8C, 8Y and 8K, which are first, second, third and fourth image forming means for different colors, are arranged substantially horizontally along a transferring and conveying belt 7 as an endless conveying means for conveying a recording medium, and a recording medium 1 is borne on and conveyed horizontally by the transferring and conveying belt 7 while an image is formed. In another configuration, as shown in FIG. 10, the first to fourth process stations 8M, 8C, 8Y and 8K are arranged vertically (substantially in the gravitational direction) along the transferring and conveying belt 7, so that a recording medium 1 is conveyed vertically while an image is formed.
For the thus constructed color image forming apparatuses to output full color images, as shown in FIGS. 9 and 10 the four color process stations 8M, 8C, 8Y and 8K, or more specifically, photosensitive drums 9M, 9C, 9Y and 9K, which are image bearing members, are brought into close contact with the conveying belt 7. Then, to form an image, a recording medium 1 is attracted by an attracting roller 18 to the surface of the transferring and conveying belt 7 so that the recording medium 1 is conveyed. Finally, to obtain a full color image, the respective color toner images are sequentially transferred to the recording medium 1.
On the contrary, when, for example, only a black image is to be formed by using only the fourth process station 8K, which is located downstream-most in the direction in which the transferring and conveying belt 7 moves, the first to the third, upstream process stations 8M, 8C and 8Y are not required, i.e., are not used for an image formation. As shown in FIGS. 11 and 12, the first to the third process stations 8M, 8C, and 8Y are halted and a separation roller 19 separates the transferring and conveying belt 7 from the first to the third process station. With this arrangement, since developing devices 12M, 12C and 12Y, of the first to the third process stations 8M, 8C and 8Y perform no unnecessary operations, the service lives of these components can be extended.
However, in the arrangement shown in FIG. 12, a recording medium 1 is conveyed substantially vertically (substantially in the gravitational direction). When only the black color image is developted, the recording medium 1 must arrive at the downstream-most, fourth process station 8K, while closely attracted to the transferring and conveying belt 7. Thus, a high voltage must be applied to the attracting roller 18 to attract the recording medium 1 to the transferring and conveying belt 7, and the cost for the power supply required by the attracting roller 18 is increased.
Further, when a recording medium 1 is rather damp or is bent, or when a recording medium 1 is formed with double sheets of paper such as an envelope, the force of attraction is deteriorated, so that the recording medium 1 tends to fall off due to its own weight to cause a paper jam.
In the above normal arrangement, the image forming procedures for which only black and for which full color is used can easily be distinguished, one from the other. However, this is not so for the case shown in FIG. 13, where the first and the third process stations 8M and 8Y output images, while the second and fourth process stations 8C and 8K do not output images. In this case, the second and the fourth process stations 8C and 8K which are not required to form images must be driven. Therefore, the developing devices 9C and 9K in the second and the fourth process stations 8C and 8K are ealy deteriorated.
It is an object of the present invention to provide an image forming apparatus that forms an image while conveying a transfer material vertically, and wherein the service life of developing means is extended.
It is another object of the present invention to provide an image forming apparatus that forms an image while conveying a transfer material vertically, and wherein the conveyance of a transfer material is ensured without an increase in cost.
To achiever the above objects, according to the present invention, an image forming apparatus comprises: a plurality of image forming means vertically arranged each of which includes an image bearing member on which a latent image is to be formed and developing means for developing the latent image; image bearing member driving means for driving the image bearing members, respectively; development driving means for driving the developing means, respectively; transfer material bearing and conveying means for bearing and conveying the transfer material; and control means for selecting image forming means to be used for image forming from among the plurality of image forming means, and for controlling the image bearing member driving means and the development driving means, so that, during an image forming process, image bearing members of image forming means that are not used for image forming among the plurality of image forming means are driven, while developing means of the image forming means that are not used for image forming are halted,
wherein, while the transfer material is vertically conveyed through transfer nips that are formed by the transfer material bearing and conveying means and all the image bearing members of the plurality of image forming means, images formed by the plurality of image forming means are sequentially transferred to the transfer material to form an image.
With this arrangement, when the developing means is not being used for image forming, the developing means is halted so that unnecessary deterioration of the developing means can be prevented, thereby ensuring that the service life of the developing means is extended.
Further, since the image bearing member that is not being used for image forming can be driven and a transfer material can be conveyed while sandwiched between the transfer material bearing and conveying means and the image bearing members, the conveyance of the transfer material is ensured without an increase in cost being required.